Color proofing is a process used by the printing industry to simulate proofs generated on a press run. By using color proofing the printing industry saves time and money simulating how the press will look before the costly press run is performed. The advantage of a color proof is that it is a representation of an ideal press run. The color proof should reflect exactly what the printing industry would like to see coming off a press. The press is continually adjusted to match the color of the proof. The color proof therefore needs to be precise color, reproducible from proofer to proofer, and pre-press shop to pre-press shop. Proofs that exhibit color variation are deemed unacceptable.
One commercially available image processing apparatus, which is depicted in commonly assigned U.S. Pat. No. 5,428,371, is an image processing apparatus having half-tone color proofing capabilities. This image processing apparatus is arranged to form an intended image on a sheet of thermal print media by transferring dye from a sheet of dye donor material to the thermal print media by applying thermal energy to the dye donor material, to transfer dye to the thermal print media, thereby forming an intended image. This image processing apparatus is comprised generally of a material supply assembly or carousel, lathe bed scanning subsystem (which includes a lathe bed scanning frame, translation drive, translation stage member, printhead, and vacuum imaging drum), and thermal print media and dye donor material exit transports.
The operation of the image processing apparatus comprises feeding a sheet of thermal media from the media roll to the vacuum drum, partially wrapped around the drum, cut to length, then wrapped fully around the drum. A length of dye donor from a roll form is similarly transported to the drum, partially wrapped around the drum, cut to a desired length, then fully wrapped around the vacuum drum. The dye donor material is wrapped around the vacuum imaging drum, such that it is superposed in registration with the thermal print media. The translation drive, part of the scanning subsystem, traverses the printhead and translation stage member axially along the vacuum imaging drum in coordinated motion with the rotating vacuum imaging drum to produce the intended image on the thermal print media.
The printhead includes a plurality of laser diodes which are coupled to the printhead by fiber optic cables which can be individually modulated to supply energy to selected areas of the donor in accordance with an information signal. The printhead includes a plurality of optical fibers coupled to the laser diodes at one end and at the other end to a fiber optic array within the printhead. The printhead moves relative to the longitudinal axis of the vacuum imaging drum and dye is transferred to the thermal print media as the radiation, transferred from the laser diodes by the optical fibers to the printhead to the dye donor material, is converted to thermal energy in the dye donor material.
Color variation is typically a result of variation of the individual color density used to define the desired color. There are many factors that influence the variation of a color in a proof. Factors include but are not limited to environment variability, density calibration technique, optical noise, thermal media coating quality, densitometer measurement noise, lamination noise, and digital proofer focus errors.
Due to factors that cause density variation, calibration of density is required to achieve high levels of consistency between a requested or desired density and the average density on an imaged proof. Calibration to the average proof density is desired due to the fluctuation of density within a single proof. To be most color accurate across the entire proof the average density should closely match the requested density.
Some early digital proofers, such as U.S. Pat. No. 5,268,708 utilized a linear calibration model over a specified density range. Specifications for the output density range were developed from printing standards. The SWOP standard was the model used for the calibration range for U.S. Pat. No. 5,268,708.
Presses are capable of producing more than cyan, magenta, yellow, and black output colors. Often special colors are added to expand the color of a print. Colors from a press are limited to the various ink colors that can be mixed or created—a virtually endless assortment. Presses therefore had a much larger color gamut than proofers, which are limited by the color gamut produced by thermal dye.
With the introduction of U.S. Pat. No. 5,428,371, color gamuts were expanded using a concept named “Recipe Color.” The amount of usable density range was increased for each thermal dye donor. Multiple passes of the same bitmap used for a single color plane were performed using different thermal dye donor material. This allowed for custom color creation, and with the introduction of a few special thermal dye donor material, the color range of a proof is extended to closely match a presses output using many different inks.
Initially the calibration of density for the extended density range of a thermal dye donor was performed by extrapolating the linear model derived using a calibration curve fit to the SWOP density range. The thermal donor material however did not exhibit a linear response across the full output density range. There existed variation in linearity between dye donor material and a manufacturing event of a single dye donor material. Furthermore thermal dye donor linear response varied machine to machine. Thermal dye donor also responded unpredictably due to machine shifts, or machine servicing.
The imaging and lamination process for U.S. Pat. No. 5,428,371 often can lead to dust spots on a proof. Additionally scanning the calibration target using a scanning spectrophotometer can lead to erroneous data. The initial calibration model for U.S. Pat. No. 5,428,371 did not filter out possible erroneous data which therefore led to inaccurate calibrations.